Zipcar is looking to sell around 6.7 million shares when it finally goes public, meaning it could raise somewhere between $93 million and $107 million excluding underwriting costs — higher than its original estimate of a $75 million IPO. Existing Zipcar shareholders are also selling around 1.7 million shares — though none of the proceeds from those sales will go to Zipcar, according to the filing.

There were some questions as to when the company would finally go public after it closed a $21 million funding round led by Meritech capital in December. That funding was designed to clean up its balance sheet and make the stock more appealing to public investors.

The service is available in most cities — cars are kept at various locations across a city in special parking spots. Users sign up for a subscription and then schedule a time and a car to pick up. They receive a card that activates the car and are free to drive it during their scheduled time.

The updated filing also indicated that Zipcar brought in $186 million in revenue in 2010, up 42 percent from $131 million in 2009. The company still lost $14 million in 2010 — compared to a loss of around $4 million in 2009. That loss comes from Zipcar’s aggressive acquisition strategy — it has bought several car-sharing services to ramp up its operations in other cities. Zipcar most recently picked up London car-sharing service Streetcar and acquired a minority stake in Spanish car-sharing service Avancar. Zipcar also acquired American rival Flexcar in 2007.

To date, the Cambridge, Mass.-based company has raised $59 million to finance its operations.